This invention relates to the melting of solid materials to produce a vitrified and/or crystalline material, by initiating a melt by passing electrical current through a planar starter path positioned between a plurality of electrodes, and thence through the surrounding solid materials. The solid materials may be soil, either undisturbed or staged, waste materials assembled at a site for disposal, or any other solid materials that can be melted and which will support joule heating during processing.
In-situ vitrification or melting of soil and other solid materials is well known, as illustrated by the many patents issued to, among others, Battelle Memorial Institute. For example, U.S. Pat. No. 4,376,598 issued Mar. 15, 1983, discloses a method of solidification of soil and other solid materials contained in the soil, by passing electrical current through melted materials between electrodes. An initial electrically conductive resistance path is provided between the electrodes, and application of current to the electrodes is continued until the solid materials between the electrodes have been melted.
The electrically conductive resistance path (the "starter path") is necessary to obtain electrical conduction between the electrodes sufficient to initially generate adequate heat to melt the soil and other solid materials adjacent to the starter path, and then to transfer the flow of current to these melted materials. Upon melting, the molten earth or other material becomes significantly more electrically conductive than it is in the unmelted state. Electricity then can flow through the molten media, being converted to heat by the phenomenon of joule heating, which heat is then conducted into and melts more adjacent solid materials. Such melting has heretofore been initiated at or near the upper surface of the solid materials in a horizontal linear path, with the melt area growing outward and downward as electrical power continues to be applied.
A number of different methods of establishing the electrically conductive resistance path have been proposed, such as graphite or sodium hydroxide paths, sacrificial resistance elements (a metal resistance coil or wire), and chemical reagents to create a highly exothermic chemical reaction. In U.S. Pat. No. 5,004,373, a cord of dielectric material (such as glass fiber) is impregnated with a conductive material (such as graphite) for initiating in-situ vitrification.
As illustrated in U.S. Pat. No. 4,376,598, the starter path was a relatively small "layer" (2.5 cm deep and 2.5 cm in width) of graphite flakes in a trench between the electrodes.
The intent of such graphite layer was merely to "provide a conductive resistance path [between the electrodes to] raise the temperature of the soil about the conductive resistance path to its melting temperature." While it was contemplated that the vitrification of materials would proceed as illustrated in the '598 patent, in fact it was found that the formation of the melt zone more closely approached that illustrated in U.S. Pat. No. 4,956,535. Starter paths having graphite as a primary component (generally in the form of flakes) are now the preferred method of initiating the melting process.
It is now known that with a horizontal, linear starter path placed near the surface of the ground as in the prior art, the melt zone progresses as illustrated in FIG. 1 herein ("Prior Art") and in FIGS. 10 and 12 of the '535 patent. As illustrated in FIG. 1, the melt 10 is initiated by a horizontal linear starter path 26 and assumes a balloon configuration, as it expands in all of the "X" (laterally in the plane between the electrodes), "Y" (downwardly"), and "Z" (laterally perpendicular to the plane of the electrodes) dimensions. The result is a melted cylindrical mass with hemispherical ends. The melt pool 10 illustrated herein represents the "growth" of the melted area as the melt pool grows. Therefore, as illustrated in FIG. 1 (and in subsequent drawings), the melt pool grows sequentially from the startup at A, and then grows downwardly and outwardly to B, C, D and E. The previous melt pools (A-D) are shown as discrete entities for the purposes of illustration only--in fact, the melt pool increases in size over time until it results in a single large melted area. Volume reduction and subsidence accounts for the location of the final melt pool and solidified mass occupying a volume substantially smaller than that previously occupied by the un-melted solid materials (the volume of A+B+C+D+E). As illustrated in the '535 patent, additional electrodes were necessary to "square up" the vitrified mass. Through experience, it has been found that conduction of heat from the melted volume into adjacent unmelted materials is directly related to the melted volume's surface area.
In theory, in perfectly dry, uniform soil, a melt will progress uniformly in all of the X, Y and Z directions. Because the melt progresses in both directions (from a line drawn between the electrodes) in the "Z" dimension, the lateral growth will theoretically be twice the downward growth, hence a theoretical aspect ratio (the ratio of depth-to-width: Y/Z) of 0.5. It is believed, however, that because most materials to be melted contain liquids or other vaporizable materials (such as water in soil) which will be vaporized by the downwardly-encroaching melt pool, the relatively "cool" vapors will proceed up the sides of the melt pool, cooling the sides and slowing the rate of lateral (Z) growth. Therefore, the rate of downward growth of a prior art melt pool will increase nominally faster than the lateral growth rate, resulting in an actual aspect ratio of up to 1.5. However, as the melt pool grows larger, other factors related to heat transfer cause the rate of downward ("Y") melting relative to outward ("z") melting to diminish (illustrated in FIG. 1 with successive melts A, B, C, D and E), and ultimately it becomes uneconomic to continue melting with the intent of extending the melt downward with conventional equipment, since the rate of (unwanted) growth in the "Z" dimension greatly exceeds the rate of desired growth in the "Y" dimension. Thus, the conventional technology is limited to operation with melt aspect ratios in the range of approximately 1.0 to 1.5.
Applicant has observed in large scale applications employing up to four megawatts of power, and utilizing the prior art linear starter path startup procedure, after a 4-electrode melt reaches about 20' in depth, with a width of approximately 40-45', the downward growth of the melt slows to a point of being uneconomic to continue (insufficient power is available to melt a much larger mass), and such melts are typically terminated at that point, or earlier. Therefore, when using commercially available large scale equipment, there is an inherent limit to how deep one can extend in situ vitrification. Of course, more powerful equipment can be used to produce even larger, deeper melts; however such equipment would have its own economic depth limit.
Applicant has found that, in using the methods disclosed in, for example, the '598 patent, the current practical limit of melt depth ("Y") using commercial scale in-situ vitrification equipment (4 MW) is approximately 20 feet. At this depth, using a starter path as disclosed in the '598 patent, the melt width ("Z") is approximately 20-22 feet (per pair of electrodes, or about 45 feet for 4 electrodes). Therefore, unless the area to be vitrified is at or above about 20 feet below the surface, it is not economical to continue melting laterally in order to minimally increase the depth of the melt. While thermal barriers may be employed to limit lateral ("Z") melt expansion, such barriers are difficult to construct, may not work properly, and are expensive.
As noted above, conventional melts having a horizontal linear starter path necessarily begin very wide and very shallow, thereby producing a very low aspect ratio (depth/width), which increases as the melt grows in depth. Applicant has observed that the aspect ratio of conventional melts rarely if ever is greater than about 1.0, or 1.3 at the maximum, at depths of commercial interest. For example, using conventional equipment, the largest practicable melt at the closest electrode separation (10') with a horizontal linear starter path therebetween, produces a melt approximately 20' wide and 20' deep per pair of electrodes.
There are many cases in which a shaped melt zone (having an aspect ratio of &gt;1.5) may be desirable. The ISV process of the '535 patent produces a melt that "grows" at will, whereas the present invention permits a melt to be tailored to fit either the site requirements and/or to reduce cost. One of the greatest advantages of being able to control melt aspect ratio is the minimization of widthwise overmelting.
For example, when it is desired to melt a volume 20 feet deep and 10 feet wide using conventional technology, it would be necessary to melt 20' wide in order to attain the 20' depth. Such processing involves melting twice the amount of material targeted, resulting in twice the time and cost. In such applications, it is desirable to perform a melt with an aspect ratio of 2.0 (twice the depth as width). The ability to control the aspect ratio can have tremendous impact on the cost of a melt and therefore its commercial viability.
As illustrated in FIG. 2, many hazardous waste sites are arranged as trenches 12 wherein the hazardous waste is buried with soil in a "U" or "V" shaped trench. The side walls of the trench may be lined with rock 14. In-situ vitrification may not be cost effective, or may present safety problems, in such cases since the natural formation of the balloon shaped melt pool 16 (as illustrated in the '535 patent) is the exact opposite shape one would desire in such cases. Because the melt may seal off to the sides of the trench, lateral movement of gases 18 generated under the melt pool 16 may be restricted by the sides of the trench, and such gases may be forced upwardly 20 through the melt zone 16, creating disturbances and discontinuities therein. Such "bubbles" may cause significant problems in maintaining an effective melt and may cause eruptions at the surface, endangering the integrity of electrodes 22 and off-gas collection apparatus 24 covering the melt. Such eruptions have been severe enough to cause melting and/or damage to hood components 24 and other equipment associated with the ISV process.
Generally speaking, in-situ vitrification now proceeds with electrodes 28 that are continuously fed (possibly through a sleeve 30) into the melt pool as the melt process proceeds (FIG. 3). As currently practiced, neither the electrodes nor the sleeves are initially inserted to the desired final depth during startup. As the melt pool grows downwardly, the electrodes are fed downwardly.
Additionally, there have been many attempts to create subterranean "walls" of vitrified material to act as barriers. Such underground structures have not heretofore been constructed economically or with precision, because the aspect ratio is so small (melts are too wide) as to make such structures uneconomic. Accordingly, a 20' deep wall made by two electrodes using conventional methods produces a 20' wide (or wider) melt--using substantially more time and energy melting than is required to produce a "wall". For barrier wall applications, it is desirable to be able to control melt aspect ratio in the range of 4 to 20, which is impossible when using prior art technology (capable of maximum aspect ratios in the range of 1.0 to 1.5).
Numerous inventions have been disclosed to aid in the practice of in-situ vitrification. For example, U.S. Pat. No. 4,762,991 discloses a probe which monitors a plurality of sensors placed along the expected path of an ISV melt. The probe receives temperature signals from the sensors and transmits them to a remote location. U.S. Pat. No. 5,024,556 discloses a system to promote destruction of volatile and/or hazardous contaminants during in-situ vitrification by forming a cold cap over the vitrified mass.